The present invention relates to placing a marking on a substrate or other object which is invisible to the unaided human eye but can be detected using an apparatus producing light in the visible and near infrared spectra. The invention further relates to a method of encoding information on a substrate using a near infrared fluorescing compound and detecting the compound.
Prevention of fraud, copying, or theft of goods such as automobiles, boats, motorcycles, bicycles, art, collectibles, and important documents such as financial instruments (stocks, bonds, security papers, checks and bank notes), and government issued papers (passports, drivers licenses and identity cards) has been a long-standing problem in society. The prior art is replete with many approaches to deter or avoid such fraud, copying or theft by placing visible and/or invisible identification marks on the goods and documents. Such identification marks have been placed openly or in covert locations. For example, U.S. Pat. No. 4,239,261 issued to Richardson on Dec. 16, 1980 discloses a micro-marking label applied to an article. The marker or label is formed from a thin plate of generally clear plastic material which is typically transparent or may be tinted with a specific color if required. The thickness of the material is about 0.001 of an inch to 0.002 of an inch (1 to 2 mills) with an overall dimension ranging from about 0.0286 by 0.0286 of an inch. The exact dimensions of the marker or label can be varied to fill space requirements or to eliminate optical resolution or optical definition limitations of photo reproduction equipment. The area of the marker is divided into segregated digital areas into which homogeneous or digital markings are placed in order to designate a specific code to identify the object. A disadvantage of this marker is that it resides on the surface of the substrate. The label can be covered up, knocked off or obliterated in which case the marker or label looses its effectiveness.
U.S. Pat. No. 5,083,814 issued to Guinta et al. on Jan. 28, 1992 discloses a security method for applying a security marking to an automobile, boat and the like. The method involves a nationwide network of authorized dealers which are supplied with input and output devices such as computer, monitor and a hand-held marking device. Using specified locations data supplied from a central process unit, the dealer applies to the surface of the automobile a confidential and invisible registration code.
In many printing technology applications it is necessary to distinguish an original from a copy or counterfeit item. With modem copying techniques, printed material can be reproduced at will and is virtually indistinguishable from the original, especially if the copy is printed on a substrate that is similar to the original. It is well known that various means and methods have been proposed for covertly marking and identifying items to circumvent and economically dissuade someone from copying the original. Typically, such means and methods used inks or paints that fluoresce when subjected to an ultraviolet light source. Such simple fluorescent marking used in conjunction with ultraviolet light provides a dramatic effect, since the marking, which is seemingly invisible to light in the visible spectra of 400 to 700 nanometers becomes brightly fluorescent under ultraviolet radiation. For example, U.S. Pat. No. 4,736,425 issued to Jalon on Apr. 5, 1988 discloses a two-step marking method for important documents, such as security papers, bank bills, checks, shares, stamps and the like to prevent forgery and to authenticate the document. In the first step of the marking process, one or more elements which form a chelate are deposited in or on the security paper. The elements are chosen so that the chelate is not formed until later. In the second step of the process, the chelate is formed by depositing on the paper the missing components to produce the synthesis of the chelate. The missing elements are added to the paper by means of an aqueous alcoholic deposition. Accordingly, it is possible to deposit the ligands in the first step and the metal ions in the second step, or vice-versa. The chelated compounds are formed with metals and rare-earth elements and are invisible under sunlight but are fluorescent when exposed to ultraviolet light rays.
This method is disadvantageous because in performing the first step, care must be taken to ensure that the deposited solutions are properly pH adjusted for the chelate when formed. Second, there must be a significant excess of the ligand to form the chelate.
U.S. Pat. No. 4,591,707 issued to Stenzel et al. on May 27, 1986 discloses the use of a hallmark on financial paper, such as bank notes, currency and the like. The hallmark is a coating on the exterior surface of the paper substrate applied by vacuum disposition techniques, such as evaporation or cathode sputtering, in the form of a pattern, stripes or figures.
U.S. Pat. No. 3,614,430 issued to Berler on Oct. 19, 1971 discloses a method electronically retrieving coded information imprinted on a substantially translucent substrate. The ink used to code the information fluoresces when exposed to ultraviolet light. The fluorescence is photoelectrically sensed through the translucent substrate. A reader device then interprets the coded information and may further preform a desired output relative to the coded information.
U.S. Pat. No. 3,933,094 issued to Murphy et al. on Jan. 20, 1976 discloses a substrate such as, a business reply envelope, having bar code information imprinted thereon. The bar code is printed on the substrate using a plurality of inks having a color which, in combination with the substrate, yields a Print Contrast Signal substantially less than 50 percent when measured in the wavelength range of 800 to 900 nanometers. Added to the ink is a metallic compound sufficient to increase the Print Contrast Signal of the ink and substrate to at least 50 percent.
U.S. Pat. No. 4,504,084 issued to Miehe et al. on Nov. 12, 1991 discloses a method for marking originals so that copies can be distinguished from the originals. The method includes using a ribbon having a printing medium for printing the original. The ribbon includes a substance in the form of a marking which, when used, produces an invisible distinguishable marking which is recognizable only by using a special scanner.
U.S. Pat. No. 5,514,860 issued to Berson on May 7, 1996 discloses a document having encoded or encrypted data printed on a transparent tape. The data is printed using an invisible ink. The ink becomes visible only when exposed to ultraviolet light or infra-red light, depending upon the dyes used in the ink.
U.S. Pat. No. 5,514,860 issued to Auslander et al. on Aug. 6, 1996 discloses a bar code having more information than a standard bar code by printing an upper layer and lower layer bar code. The ink used to print the lower bar code is a regular ink which absorbs in the visible range of the spectra, i.e., between 400 and 700 nanometers. The upper layer bar code is printed using an ink that is invisible to the naked eye. The invisible inks used are based on complexes of rare earth elements such as Eu, Th, Sm, Dy, Lu and various chelating agents to produce chromophore ligands that absorb in the ultraviolet and blue spectra region. The lower bar code is read by a first excitation source emitting a first wavelength and a first sensor and the upper layer bar code is read by a second excitation source emitting a second wavelength and a second sensor.
In all the systems that use fluorescence to highlight the encoded markings it is required that there be a reading system or scanner that will excite the fluorescing compounds. The fluorescing light is then reflected back to a detector, collected and translated into an electric current signal that is proportional to the light intensity and the area illuminated. The electric signal is then decoded and used in a predetermined manner. A disadvantage of these systems that use an ultraviolet fluorescing ink is that many materials and substrates, such as dyes, paints and coatings, have brighteners or other compounds that substantially interfere with the ultraviolet light absorption. Moreover, the ink or encoded markings must be placed close to the surface of the substrate. Thus, the markings are highly susceptible to being easily scratched or detected using a readily available ultraviolet scanner.
Therefore, there is a need for an invisible marking that can be placed on a substrate that can be substantially hidden from visible and ultraviolet light detection yet still provide a means for determining authenticity of the document. There is also a need for an invisible marking that can be placed on a substrate and protected from being inadvertently rubbed off, scratched or removed.